Патент USA US3045718код для вставки
July 24, 1962 G. E. ZIEGLER 3,045,708 HEAT DISTRIBUTION SYSTEM AND METHOD OF MAKING SAME Filed Jan. 12, 1959 3 Sheets-Sheet 1 I A 25\ \ \\ \\\ \\\ \ /25 ‘ \ \\\m» \\\\\\\\ \\ \' \ \\ \\\ \\\ >\\ r \22 LA \24 GEORGE ‘E. Z‘EG LE2 INVENTOR. July 24, 1962 G. E. ZIEGLER 3,045,708 HEAT DISTRIBUTION SYSTEM AND METHOD OF MAKING SAME Filed Jan. 12, 1959 3 Sheets-Sheet 2 FIG- 7 TEMPRAU 500 700 600 500 o \ ’L a 4- 5 e ‘I INCHE5 FROM HEATED PIPE. PEMTEK4AT5U F l G. b 12.5456159l0?l2? INCHES FROM HEATED PIPE SURFACE 55° mca PIPE D\AME.TEIL 55o ' E 300 BFLPtIOeN0uEr5AL.T Fla- 9 250 200 I50 IO 20 30 4-0 50 60 TO 50 DAY‘E: GEORGE ‘E; 'ZlEGL-EK INVENTOR. July 24, 1962 G. E. ZIEGLER 3,045,708 HEAT DISTRIBUTION SYSTEM AND METHOD OF MAKING SAME Filed Jan. 12, 1959 3 Sheets-Sheet 3 FIG. 11 'FlC-L 14 F\G-15 GEORGE E.ZIEGL:ER INVENTOR. United States Patent 0 ” IC€ 3,045,708 Patented July 24, 1962 0 l 7 3,045,703 a _ , _ . , HEAT DISTRIBUTION SYSTEM AND METHOD OF MAKING SAME George E. Ziegler, Evanston, Ill., assignor, by mesne as signments, to Concrete Thermal Casings, Inc., Seattle, Wash., a corporation of Washington Filed Jan. 12, 1959, Ser. No. 786,169 5 Claims. (Cl. 138-106) . in insulation. Thus a modest amount of residual moisture can cause a signi?cant fuel dollar loss over a period of years. It is therefore a ?rst and principal object of my inven tion to protect heat distribution systems from water de terioration by continuously removing water and water vapor from the system at a rate approaching or even ex ceeding the average rate at which water enters the system. A further and principal object of my invention is to The present invention relates to improved heat distribu 10 make it possible to operate heat distribution systems with improved thermal performance without the use of pro tion systems and more particularly to systems having in tective casings surrounding the insulation. creased resistance to deterioration by water and ‘a method A further object ‘of my invention is to make possible for producing same. the economical operation of heat distribution systems with Heat distribution systems usually consist of one or semi-permeable casings surrounding the insulation. more heated ?uid-carrying pipes surrounded by suitable Another object is to remove water from the thermal thermal insulation. My present invention is applicable to insulation by direct drainage. underground systems in which the pipe and insulation Another objective is to remove water vapor from the are buried in the ground, and to overhead systems in which the pipe and insulation are supported in ‘air by suitable ' thermal insulation by evaporation. Another object is to minimize the entry of water by in structures either above grade or in tunnels. vIn both classes 20 ?ltration from ?ooded ground or from rain in the case of systems it has been customary to protect the insulation of overhead systems. from-in?ltration of water from the ground and from rain A further objective is to provide a path of maximum by surrounding the insulation with a casing which may be resistance to the ?ow of water through the insulation. metallic or nonmetallic. The heated ?uids most com monly encountered are hot water, steam, high temperature 25 A still further objective is to provide pipe support when such support is needed ‘as well as providing for such re (high pressure) hot water and heated oils. quirements as thermal insulation, the inhibiting of cor In the past, efforts have been made to improve the rosi-on of the pipes and associated metal supports, and water migration resistance of the thermal insulation and in the case of underground systems, to protect the pipes to devise watertight'casings; nevertheless, a recent author itative engineering study reports that in heat distribution 30 from mechanical forces transmitted through the earth. Many and further objects of the invention as well as systems now in operation, water is the major adverse fac advantages and features thereof will be ‘apparent from tor encountered and that all systems eventually become the discussion of the invention which follows, and it will wet.- Water enters the distribution systems from internal be understood moreover, in said discussion which more pipe breaks, from in?ltration from the ground, from ?ood~ speci?cally describes the invention, that the same is not ed manholes, or from rain leaking through the casings, depending on the class of the system. . My invention is applicable to ?eld-fabricated poured in-place heat distribution systems. These include systems of the Golf type, Patent Number 2,355,966, in which the. pipes are supported by the insulating concrete, as well as systems in which the pipes are supported on conven tional guides, rollers or rockers, and the insulation con to be taken in a limiting sense but merely as illustrative of the invention the metes and bounds of what is to be considered patentable therein being de?ned by the ap pended claims. A fuller understanding of the invention may be had by referring to the following description taken in conjunction with the accompanying drawings in which: sisting of insulating concrete or bituminous material which is fused or solidi?ed by heat serve only thermal purposes. FIGURE 1 shows a longitudinal cross section of a and installed in tunnels or above grade. FIGURE 3 is a cross sectional view ‘of a heat distribu short length of heat distribution system with evaporation My invention is also applicable to heat distribution‘ 45 and drainage channel. FIGURE 2 is a cross sectional view taken along the line systems utilizing preformed or sectional insulation placed A--A of FIGURE 1. on pipes supported on conventional supports and guides tion system with multiple evaporation channels. Heat distribution systems require protection from tWo types of di?iculty with water. The ?rst of these dii?cul 50 FIGURE 4 illustrates cross sections of four shapes for ties is the occasional ?ooding of the insulation from major evaporative channels. breaks in the ?uid distribution pipes, from unusual ground water ?ooding, from back pressure of water through the zation of water. insulation caused by ?ooded manholes or from severe rams. . In this ?ooded condition the system can not be satisfac torily operated because of attendant violet boiling action within this insulation. This boiling action is disastrous to the physical structure of the insulation as well as proa hibitively expensive as far as heat loss is concerned. The boiling action can also result in pipe corrosion. FIGURE 5 shows a concentric channel for the vapori . FIGURE 6 shows a modi?ed concentric channel for theivv-aporizdation of water. FIGURE. 7 is a graph of insulation temperature as a function of distance from the surface of a 350° F. heated Pipe FIGURE 8 is a graph ofinsulation temperature ‘as a function of distance from the surface of a 750° F. heated pipe. , The second di?iculty is much less obvious than the ?ood FIGURE 9 is 1a graph of thermal loss of heat distribu ed state with its boiling action. However, it is equally tion system. ‘ Y , FIGURE 10 is apparatus for measuring water migra~ serious especially since the condition is apt to prevail dur~~ _ , ing the entire operating ‘life of the heat distribution sys 65 tion resistance of insulation. tem. This second difficulty is the more or less permanent FIGURE 11 is a cross section of sectional preformed reduction in insulation performance caused by the presence insulation equipped with evaporative' channel. ‘ FIGURE 12 shows a multiple pipe system with mul of moisture in the insulation. It is well known that the insulating values of all materials diminish rapidly with, tiple ev-aporative channels. ‘increasing moisture content. A one percent increase in 70 FIGURE 13 shows water migration paths and isother— mal curve in a heat distribution system with casing and moisture content will ‘result in from two to four percent evaporative channel. increase in thermal conductivity, with a corresponding loss 3,045,708 3 A. general the possible difference in level of the channel 22 tion paths for a multiple pipe system. inside the insulation is small compared to the normal FIGURE 15 shows water migration paths for water variations in elevation introduced along a run of pipe in from break in pipe. .. , the average distribution system. The variation in eleva The general aspects of the operation of my invention 5 tion is needed to provide for internal drainage of the heat can be most easily understood by considering it in rela ed ?uid distribution pipes or to take care of variations tion to past practices. In the past, attempts have been in terrain. made to minimize the entry of water into the heat distri Even after ?ood water has been drained off through bution system through the use of an external casing to cocks 21, the moisture content of the insulation remains keep out the water, and by waterproo?ng admixes added high enough to cause excessively high heat losses. In to the insulation to reduce the rate of migration of any my invention as illustrated by FIGURE 1, this moisture water which might accidentally enter through the casings is evaporated from the insulation into channel 22 and re or come from breaks in the heat distribution pipes. Since moved from the channel by air which circulates either the casings surround the insulation and are essentially as natural draft resulting from a thermal syphon action FIGURE 14 shows isothermal curve and water migra continuous and relatively impervious to water vapor, no 15 in the vertical vents 27 and 28 or by induced draft from appreciable evaporation of water can take place. In fact, water accumulated in the insulation because the quantity of water as liquid that was drawn into the insulation by temperature changes through accidental openings in the a fan 26. The intake pipe 27 and the exhaust pipe 28 are protected from the entry of rain by any conventional type hood of which the inverted U 29 is one example. The channel 22 may communicate at its ends to free or casing in a given length of time is larger than the amount 20 open air or it can open into ventilated manholes or base ments. For long runs of pipe a multiplicity of outlets ing in an equal time. In sharp contrast to this situation, and inlets can be used as a means of increasing the rate my invention makes it possible to shift at will the balance of evaporation. These are design variations which all of Water that can leave as vapor through the same open between water intake and water vapor outgo so that the come within the scope of my invention. system on the average over an extended period of time In general the rate of evaporation of water from the insulation into the channel 22 is dependent on the aver age temperature and the percentage of moisture content will dry out instead of accumulating water. In addition to providing for the evaporation of water, my invention provides for the drainage of excess or ?ood of the insulation surrounding the channel. water from the insulation. FIGURES 7 and 8 show typical temperature gradients In the past it has not been possible to use insulation 30 through insulations containing moisture for a direction without casings in systems exposed to the weather above B—B of FIGURE 2, with distances measured out from grade or buried underground systems because the mois the surface of the insulation for a section of insulation ture content of the insulation increased too rapidly to as shown in FIGURE 2 for two different pipe tempera constitute a satisfactory system. My invention causes the tures and two different insulation thicknesses. removal of water vapor to be rapid enough so that the 35 The moisture level in the insulation depends on a num moisture content of the insulation will not build up to ber of factors. An important factor is the resistance of an objectionable level, thus making it possible to operate the insulation to the ?ow of water. The resistance of systems constructed Without casings. ?ow or migration is different vfrom capillary absorption This invention represents an improvement over the pipe into dry material which is often erroneously measured insulation systems broadly disclosed and claimed in the 40 to give an indication of permeability of water soaked ma applications of Lincoln L. Loper, Jr., Serial Nos. 689,584 terials. With increasing resistance, the rate at which now abandoned and 9,03 6. water is added to any section of the insulation becomes My invention introduces the idea of controlled rate of smaller and it is easier to overbalance the water intake evaporation by placing an evaporation channel at a loca with the evaporation or vapor outgo. tion in the body of the insulation where the temperature Water migration resistance can be measured with the 'Will be appropriate to produce a rate of evaporation apparatus shown in cross section in FIGURE 10 in which greater than the rate at which water can enter the insula a six inch diameter, twelve inch high cylinder 30, of the tion either from small leaks in the heated ?uid pipe or insulation provided with a cylindrical cavity two inches from in?ltration from outside the system or both together. in diameter and eight inches deep, is immersed ten Details of this will be presented through the speci?c 50 inches in water 32 contained in a closed vessel 33. The examples of my invention. resistance can be measured in terms of water ?owing into It should be pointed out that a small thermal loss is the cavity 31 and then expressed as volume of water per caused by- the evaporation of the water from the insulation unit of time per square inch of cross section of insula tion. and from the warming of the air which circulates to re move the water vapor. This loss however is negligible In speci?c examples of my invention to be discussed compared to the relatively large thermal conductivity loss later, insulation will be characterized as being of low re that can take place from the heated pipe to the surround sistance if an appreciable amount of water accumulates ing ground or air if the insulation does not remain dry. in the cavity in less than one hour. Insulations will be With reference to FIGURES 1, 2, 11 and 13, the evap characterized as being of intermediate resistance if it oration channel which is a principal feature of my inven requires a day to a week for any appreciable amount of tion is identi?ed by the reference character 22, and is water to accumulate in the cavity 31. High resistance shown located within the insulation 25, between the heat insulation is characterized by the ‘fact that no appreciable ed ?uid conducting pipe 23 and the outer boundary of moisture accumulates in the cavity 31 even after two the insulation 24. Other examples are identi?ed and dis Weeks under test. It should be pointed out that even with cussed in connection with other ?gures. > The present invention corrects the di?iculties cause high resistance insulation, the surface of the cavity will be sensibly damp because no evaporation takes place by accidental ?ooding of the insulation by permitting the from the surface of the cavity because the air in the insulation to be drained by the drain cocks 20 and 21 of FIGURE 1. The drain cocks 20 and 21 in actual prac tice might be replaced by any type of trap or sump with automatic pump depending on the type of service. closed vessel 33 and in the cavity 31 has a relative humid ity at or near 100% saturation. Although engineers would intuitively place at least one vapor permeability so that any location in a body of in sulation can gradually dry if a means is provided for removing water vapor more rapidly than water enters of the channels such as 22in the lower part of the insula Even though insulation has high resistance to the mi gration of liquid water, it usually has sufficient water tion, I have found that satisfactory ?ood drainage takes place for any location within the insulation, because in 75 as liquid or vapor. 3,045,708 6 The moisture level in the insulation is also a function of the temperature. If the heated ?uid conducting pipe 23 has if the channel 22 were at a cooler location in the outer zone. a temperature above the toiling. point ‘of water at at The most desirable location within the insulation for mospheric pressure, then the insulation between the Sur the evaporation channel 22 can be determined on the face of the pipe 23 and the place in the insulation where Ul basis of the following considerations. the temperature is equal to that of boiling water will be If the insulation has high resistance to the migration essentially oven dry. Any free moisture in this zone of of water, then channel 22 can be placed far out in the the insulation will be driven by the thermal gradient to outer zone where the temperature will be a minimum. the place where the temperature is below the boiling This location reduces thermal losses by the air currents point of water. ?owing through the channel. The temperature gradient for a four inch diameter pipe If the resistance to migration of water is intermediate, operating at 350° F. surrounded by six inches of in then the channel 22 must be placed near the 210° F. sulation of the con?guration shown in FIGURE 2 is zone to achieve maximum temperature for the conse_ given in FIGURE 7 which is a plot of temperature in quent more rapid evaporation. With insulation of low degrees F. vs. distance from the pipe surface in inches. 15 water migration resistance, it will not be possible to evap The temperature gradient for a similar eight inch diam orate water as fast as it can run into 22. In this case eter pipe operating at 750° F. and surrounded by twelve channel 2?..would merely serve as a drain through the inches of insulation is given in FIGURE 8. drain cocks 20‘ and 21, unless a casing is used to keep out In the preferred forms of the present invention, the the bulk of the water, in which case my invention can channel or vent passage 22 is located at ‘a point inter 20 function with low resistance insulation. mediate the outside surface of the insulation and the The migration of water through the insulating con point within the insulation where the temperature is at crete constituting an underground heating distribution the temperature of boiling water. This region has ‘been conduit can be viewed analogously to the ?ow of elec referred to as the outer zone in the succeeding discus tricity through a shaped conductor. sion, and the inner zone referred to subsequently extends The resistance of the insulating concrete to the migra from the ‘outer surface of the pipe 23 to the aforemen tion of the water is equivalent to the electric or ohmic re tioned point in the insulation where the temperature is sistance. The system can be considered to be a balanced Within the outer zone as defined circuit in which the water entering and passing through above, the preferred location for the vent passage for the solid portion of the concrete is equal to the vapor steam carrying systems will normally ‘be in the range 30 evaporation. The water movement from the external from 0.3 to 0.9 times the distance from the outermost surface to the air space, is dependent in part on the total pipe surface to the outermost boundary of the insula resistance. If the resistivity of the solid part of the sys tem is relatively high, a tolerable flow of water can be tion. More speci?callyrwith steam carrying pipes operating obtained without casings or with a relatively low resist that of boiling water. at quite high temperatures (as exempli?ed in FIGURE ance coating or casing. For example, a wash of Port 8) a convenient location for the channel 22 is at the land cement ‘base waterproo?ng paint can be used as a area where the temperature is about 210° F. ‘For lower casing. Numerous varieties of such waterproo?ng paint ‘are commercially available as cement paint to waterproof temperature work, it may be located ‘at an area having a temperature of about 170° F. cement blocks and basement walls. If desired, an addi In the insulated pipe system for which the temperature 40 tional layer of water resistance can be added by applying gradient is given in FIGURE 7, the inner zone is approxi over cement paint a bituminous mopping. Sufficient re mately the ?rst two inches going ‘out from the pipe. sistance can also be obtained with the bituminous coating The outer zone is from this two inch position out to alone, i.e., a water emulsion type asphalt coating can be the surface of the six inch thick insulation. In the ex placed on the outer surface of the insulation. ample of FIGURE 8 the inner zone is approximately the If the air current circulation through the duct is small, ?rst seven inches out from the surface of the pipe. The 45 then the total resistivity must be relatively large. How outer zone is from seven inches ‘out to the surface of the ever, if a reasonable quantity of moisture is evaporated, twelve inch thick insulation. The exact location of the then a lower resistance can be tolerated and it is possible 210° F. zone in any given system ‘will be a function of to get along with the resistance of only the solid part of the insulating concrete. the moisture content of the system. More precisely the 210° F. zone is de?ned as the 210° F. isothermal sur 50 FIGURE 3 shows the cross section of a speci?c example face within the insulation and its shape depends on the of my invention which was built to obtain the thermal exterior shape of the insulation and the size and con?gu efficiency data shown in FIGURE 9. Insulating con rations of the heated pipes. This isothermal is illus crete of high water migration resistance was poured trated in FIGURES 13 and 14 by the dotted line 47. around a four inch pipe coated with a parting agent to As the insulation dries out the 210° F. zone moves out 55 prevent adhesion of the concrete to the steel pipe. Two ward and becomes narrower. The width of the 210° F. evaporation channels 35 are located in the outer part of zone is illustrated in FIGURES 7 and 8 by the essentially the defined outer zone. horizontal part of the curve at approximately 210° F. High water migration resistance insulating concrete At ?nal dryness it will be quite narrow and occupy a was made with the following ingredients: position proportionally located between the temperature 60 of the pipe 23 and the temperature of the outer surface Portland cement of the insulation. As an average for 350° F. heated ?uid systems the inner zone or above 210° F. zone oc Expanded vermiculite (small particle size, 94 lbs. waterproofed by method of Sucetti Patent cupies approximately the ?rst one-third to one-half of the distance out from the surface of the pipe 23. The Emulsi?ed asphalt ______________________ _. 3 gallons outer zone constitutes the remaining distance out. Water ________________________________ _. 21 gallons The moisture content of the insulation in the inner zone is negligible because any free moisture present would boil off or migrate to the cooler outer zone of the insulation where it condenses. Thus the channel 22 when located in the inner zone re ceives only the limited amounts of water that can diffuse from the outer zone back into the channel 22. Also the temperature losses due to circulating air are larger than 75 No. 2,355,966) ____________ __' ________ __ 6 cu. ft. The wet density of this mix was 60‘ lbs./ cu. ft. A layer of insulating concrete was poured approximately three inches deep or up to the level of the line C-C in FIG— URE 3. A U shaped metal strip was pressed into the concrete to form U shaped grooves 37 of FIGURE 3. The shaped channel was located so that approximately one inch of insulation was present on the outside of the channel. 3,045,708 7 After the concrete hardened the U shaped metal chan was ?ooded with water. nel in the concrete, thus forming an approximate oval with the surface of the lower half of the oval being con crete and the upper half metal. The rest of the insulation shown in FIGURE 3 was It should be pointed out that this system hasno casing. Air continued to circulate through channels 35 thus removing water by evapora nel was removed and inverted over the U shaped chan tion. 0 The system was left in the ?ooded condition for seven days. During this time no change in the heat loss was observed, indicating that the circulating air current poured leaving the metal inverted U forms in place. The was able to carry away water vapor at the same rate exact shape of the oval is of no importance to my in that liquid water was entering the insulation, At the end of the seven day ?ooding period the water FIGURE 4 shows a few convenient shapes 33, 39, 4t} 10 was removed and the system continued in normal opera vention. 1 tion for ten more days. At this period the heat loss was down to 205 B.t.u. The system was again ?ooded for ten days and the heat loss remained steady at 205 B.t.u. At the end of this ten day ?ooding the water was removed and 41 of channels that can be of any convenient con struction material. If the material is permeable to water it increases the evaporation area. With any of the shapes illustrated in FIGURES 2, 3 and 4, it is desirable that the channels have a cross sectional area of less thn 15% of the cross sectional area of the insulating concrete, and and normal operation continued. With thirty days of additional normal operation the preferably less than 10% of that ?gure. heat loss reached an equilibrium value of the heat loss at 148 B.t.u. The system was again ?ooded and no ap With appropriate choice of metals in regard to the electro chemical series for metals and proper electrical connection between the pipes and the forms these in preciable change in efficiency was observed. These data verted forms can serve the supplementary function of as a function of days of operation. The flooding periods providing electrolytic corrosion protection for the pipes are evident as the horizontal part of the curve. contained within the insulation. To this end, the metal of the channel should be electronegative with respect to the metal of the heat conducting pipe. The ends of the channels 37 and 38 communicated to the atmosphere with stand pipes, one pipe preferably longer than the other to aid syphon action circulation of air. Another embodiment of my invention is shown in FIG URE 12 in which four heated pipes are contained in a are presented in FIGURE 9 where heat losses are plotted single unit of insulation. Two evaporative channels 45 and 46 are employed to carry off the water vapor. Still another embodiment of my invention is shown in FIGURE 13, where a single heated pipe 23 system is shown equipped with a single evaporative channel 22 placed in the top section of the insulation. The insula tion in this system is given protection in the form of 21 Portland cement paint ‘outer casing 48. The bottom of There are many additional ways in which experienced construction workers can introduce the channels. Of these possible ways, the following serve as illustrations: (1) the channels might be formed by tubes made of stiff water permeable materials such as cardboard, ?ber, plasters, screening, ceramics or perforated metal. The tubes are fastened at the appropriate place in the form used to hold the insulation during the time required for setting. The joints between tubes are taped to exclude the wet concrete and the tubes are left in place. (2) In the production of preformed factory-made sec tional insulation, the channel can be formed by a man drel in the mold so arranged that it can be removed after the insulation takes shape. FIGURE 11 shows a cross section of one example of such a preformed section. Naturally it is necessary to cement the ends of the sec tions together to avoid the possibility of excessive move ment of water into the insulation along the radial joints between the sections. The preformed factory made sections can be made solid as shown in FIGURE 11, in which case they are slipped on endwise over the end of the heated ?uid con ducting pipe or the sections can be split longitudinally in the manner that is conventional for preformed rigid sec Either channel 45 or 46 or both can serve as the drain channel. ' this system consists of a structural concrete base pad or In this case an insulating concrete of inter mediate resistance to water migration can be used be cause the cement paint limits the in?ltration of water. I foundation. Cement paint is a typical example of a semipermeable casing. Bituminous coatings could be applied instead of the cement paint or on top of the cement paint. In general, a small leak in an otherwise completely im permeable casing will cause the system to operate the same as with a semipermeable casing. The water enter ing the casing through a small hole will distribute itself by capillary action between the casing and the insulation. It is usually not convenient or practical to achieve a non capillary bond between the insulation and the casing. In FIGURES 13 and 14 typical ?ow lines for water in ?ltrating from the outside are shown. FIGURE 15 shows the less frequent, but important case of the ?ow lines from a break in the heated ?uid pipe. FIGURE 15 also shows my invention applied to a system having an tional insulation. In this case channel 22 can be con tained entirely within one of the sections or it can be impervious casing 49. placed in the ‘longitudinal boundary between the sections factory-made section of insulation in which the evapora tive channel has been enlarged to the place where it is a concentric ring within the body of the insulation. In this form, it is desirable that the air space 22 constitute about so that one side of the channel is formed by one half of the sectioned insulation and the other side of the channel is formed by the other half of the sectional insulation. The effectiveness of my invention can be demonstrated through heat loss measurements with the system illus trated by FIGURE 3. FIGURE 9 shows the measured heat loss as a function of time. FIGURE 5 shows the cross section of a preformed or 5 to 50% of the space between the outer surface of the pipe 23 and the outermost periphery of the insulation, and that the outer cylinder of insulation, extending from the outer peripheral boundary of the air space to the outermost periphery of the insulation constitute about 30 to 95% of said space. Initially, poured in place insulating concrete has a large moisture content which results in a relatively poor 65 FIGURE 6 shows another possible con?guration which initial thermal e?’iciency. A four inch diameter pipe op is possible with either job formed or factory formed erating at 350° F. is considered e?cient when the heat insulation. The ‘factory formed section illustrated in FIGURE 11 is particularly simple and easy to fabricate. losses are about 200 B.t.u. per hour per lineal foot of system. It is also easy to cement together the ends of the sec After the completion of an initial warm up period, the 70 tions to achieve water tight joints. FIGURE 3 example showed a heat loss of 364 B.t.u. per While there are above disclosed but a limited number lineal foot per hour. Continued operation with normal of embodiments of the structure, process and product of thermal syphon circulation of air through channels 35 the invention herein presented, it is possible to produce resulted in a lowering of the heat loss so that at six days still other embodiments without departing from the in the loss was 241 B.t.u. At this time the whole system 75 ventive concept herein disclosed, and it is desired there 9 3,045,708 10 fore that only such limitations be imposed on the ap pended claims as are stated therein, or required by the prior art. The invention claimed is: 5. In a heat distribution system comprising ‘an im pervious pipe arranged tocarry a heated ?uid and a monolithic jacket of lightweight, moisture vapor per meable insulating material thermally insulating said pipe 1. ‘In a heat distribution system comprising an im about its entire periphery from a colder surrounding en pervious pipe arranged to carry a heated ?uid having a temperature in excess of the boiling point of water and a monolithic jacket of lightweight, moisture vapor perme vironment having a temperature less than the boiling point of water at the prevailing atmospheric pressure, said insulating material having at least one longitudinally ex tending vent passage formed in said insulating material about its entire periphery from a colder surrounding en 10 in spaced relation to said pipe, said vent being in com vironment having a temperature less than the boiling munication with free air, the improvement comprising point of water at the prevailing ‘atmospheric pressure, providing said vent passage in the region of insulation said insulating material having at least one longitudinally where the temperature of the insulation is intermediate extending vent passage-formed in said insulating material the temperature at the outside surface of the insulation in spaced relation to said pipe, said vent being in com and the temperature inside said pipe, and in which said able insulating material thermally insulating said pipe munication with free air, the improvement comprising vent passage has a cross-sectional area of less than about providing said vent passage in the region of insulation 15% of the cross-sectional area of said jacket. Where the temperature of the insulation is intermediate the temperature or said environment and the temperature References Cited in the ?le of this patent of boiling water. 20 UNITED STATES PATENTS 2. The system of claim 1 in which the center of said 384,860 1,991,455 vent passage is located at a distance in the range from 0.3 to ‘0.9 times the radial dimension of the jacket meas ured from the outermost pipe surface to the outermost Meehan ____________ __ June 19, 1888 Gottwald ___________ __ Feb. 27, 1931 2,081,867 Gysling ____ __. ______ __ May 25, 1937 25 2,355,966 God _______________ __ Aug. 15, 1944 3. The system of claim 1 in which the center of said vent passage is located at a point Where the temperature of the insulation is about 210° F. 4. The system of claim 1 in which the center of said vent passage is located at a point where the temperature 30 of the insulation is about 170° F. 2,360,067 McLeish ___________ __ Oct. 10, 1944 2,758,082 2,820,480 2,896,669 Rohrman ____________ __ Aug. 7, 1956 O’Rourke et a1. ______ .._. Jan. 21, 1958 Broadway et a1. _______ __ July 28, 1959 periphery of said jacket.